1. Field of the Invention
The present invention relates to a mobile communication terminal equipment mountable in automobiles and other vehicles.
2. Description of the Prior Art
A variety of mobile communication systems have been developed. The mobile communication systems can be roughly classified into satellite and terrestrial mobile communication systems.
The satellite mobile communication system is one that performs the communication among a plurality of mobile stations through an artificial satellite (and a ground base station). The satellite mobile communication system is usually organized by mobile stations mounted on various vehicles, a base station located on the ground (ground station) and an artificial satellite for wirelessly connecting the mobile stations and the base station. As such a communication system, there are known, for example, a system regulated by INMARSAT Standard M and a system INTELSAT TDMA/DSI (Time Division Multiple Access/Digital Speech Interpolation). Since almost all of the satellite mobile communication systems being currently used or planned utilize digital communication, coded voice signals, digital data and other digital signals will be handled in transmission and reception. The INTELSAT TDMA/DSI system is regulated in "Intelsat TDMA/DSI System Specification IESS-307-Rev. B", Mar. 12, 1991.
The terrestrial mobile communication is one that can make the communication among a plurality of mobile stations through a ground base station and that is usually organized by mobile stations mounted in a variety of movers and base stations located on the ground. A typical terrestrial mobile communication system is a cellular system. The cellular system comprises a plurality of base stations which are arranged on the ground in a regular pattern. Each of the base stations covers a region having a given area (e.g., a circular region having a radius ranging between a few kilometers and 10 kilometers and usually being called a "cell"). Each of the base stations uses a radio frequency different from those of the other base stations covering cells which are adjacent to the first-mentioned cell. Such a system may use the minimum number of radio frequencies and more effectively utilizes the frequency resource. A typical cellular system is an automobile phone system. The cellular systems currently used are mainly of analog type, but may be shifted to digital type in the near future. A typical analog type cellular system is shown in "Cellular Mobile Telephone Equipment Specification", May 1983, Advanced Mobile Phone Service, Inc. while a typical digital type cellular system is described in "EIA/TIA Project Number 2398", Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard IS-54-B Revision B, Jan. 25, 1991.
The cellular system has a disadvantage in that it cannot be used in some areas or regions. For economy, there are areas or regions which do not have any base station and so do not belong to any cells. For example, a base station will not be provided in a country area having a low population density, a mountainous district or a depopulated area. In such an area or region, it is difficult or impossible to utilize the cellular system.
One method of overcoming such a problem is to mount a mobile station for cellular system with a mobile station for satellite mobile communication system in a single vehicle. If the vehicle runs in an area or region in which the cellular system can be utilized (generally, in an urban area), a user (or operator) in the vehicle can select either of the cellular or satellite mobile communication system as required. In an area or region which does not belong to any cell in the cellular system, the user cannot utilize the cellular system, but can select and use the satellite mobile communication system.
However, the provision of both the cellular and satellite mobile stations in a single vehicle results in an increased scale of the entire system.